During hydrocarbon refining and processing, large quantities of water are used for many difference purposes. For example, water is used: (1) for heating and cooling; (2) for generating steam; (3) to wash hydrocarbon streams; and (4) to wash down spills. All of this water must be treated in waste water treatment facilities or systems before the water can be safely reused in the refinery or re-introduced into the environment.
Refinery waste waters are generally contaminated with a variety of undesirable chemicals agents that must either be removed or rendered harmless before the water can be reused in or released from the facility. A particularly dangerous and frequently encountered contaminant is hydrogen cyanide (HCN) or its salts, collectively referred to as cyanide.
Typically, waste waters contaminated with volatile contaminants such as HCN and hydrogen sulfide are initially subjected to steam stripping which removes some of the volatiles, especially hydrogen sulfide, from the waters as an overhead stream. However, the water effluent stream from the steam stripper, commonly referred to as sour water stripper bottoms, still contains unacceptable levels of contaminants including cyanide.
Generally, the remaining cyanide is reduced or removed from the sour water stripper bottoms by either oxidizing the cyanide or complexing it with transition metals to form cyano-transition metal complexes. Numerous patents describe these and similar methods for cyanide reduction or removal including the following representative patents: U.S. Pat. No. 4,822,496 issued Apr. 18, 1989 to Griffiths et al.; U.S. Pat. No. 4,419,247 issued Dec. 6, 1983 to Tenny; U.S. Pat. No. 4,312,760 issued Jan. 26, 1982 to Neville; U.S. Pat. No. 5,093,007 issued Mar. 3, 1992 to Domvile; U.S. Pat. No. 5,137,642 issued Aug. 11, 1992 to Castrantas et al.; and U.S. Pat. No. 4,743,381 issued May 10, 1988 to Bull, herein incorporated by reference.
However, these methods typically involve treating the waters with strong oxidants and/or various transition metals; each method has its own drawbacks. Strong oxidants require special auxiliary equipment for generation, handling and introduction of the oxidants into the waters. Such equipment is expensive and subject to breakdown and to corrosion by the oxidants. Transition metal complexation may affect metal concentrations in waters being released into the environment or reused in facility. Thus, it would be highly desirable to have a method for removing or rendering harmless cyanide that does not require expensive auxiliary equipment and does not utilize highly reactive oxidants or require transition metals.